She also described hearing a scream - "when a child hurts themselves there's a scream and it felt like a scream, and it was quick and it was high pitched and it was sharp" - but then thought she had imagined it or that it was a bird because she couldn't see any proof of her son.

Several neighbours took to the witness box to describe their desperate search for the missing boy, with residents looking through their own homes and neighbours' yards.

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Neighbour Lydene Heslop said she put a post on Facebook saying a child was missing, leading several residents of Kendall to come and join the search.

William's foster-parents were "in a very, very distressed state; they were shouting and screaming off their heads for William," neighbour Peter Crabbe told the inquest.

"I was driving to Port Macquarie Airport to collect a family member and had this flash of [realisation that] there were two cars," she said, adding she had forgotten about it in the panic of William's disappearance.

"I just went, 'There were two cars there.' My heart just sank because I thought those two cars were there for both of them," she said.

"I got back to the house, I went straight down to the command post and I told the guy on duty. He gave me his phone and said search for the cars."

William's disappearance sparked one of the state's largest police investigations to date, which has uncovered a group of North Coast paedophiles, investigated 600 persons of interest and combed through more than 15,000 pieces of evidence.

Nobody has been charged over his disappearance.

Detective Chief Inspector Gary Jubelin, who was until recently the officer in charge of the investigation, told reporters last year that his team had concluded his disappearance was most likely the result of "human intervention".

The inquest returns on Wednesday morning, with the foster-father expected to take the stand.